1. Field of the Invention
The present invention relates to a projection-type display apparatus that projects and displays an image on a screen.
2. Description of the Related Art
The recent trend is to provide a high-brightness light source in order to realize high brightness in a projection-type display apparatus, in which a liquid crystal panel or a reflective optical modulator (a reflective liquid crystal display or the like) is employed as an image displaying device.
An extra-high pressure mercury lamp, a metal halide lamp, or the like has been used as a light source for the projection-type display apparatus. Because these types of lamps have a shorter lifetime, a maintenance work to replace them is required frequently. Furthermore, when these types of lamps are employed, an optical system that takes out red, green, and blue from white light of the lamp is required. Thus, the configuration of the apparatus is complicated and the light use efficiency deteriorates.
To solve these problems, a laser source, such as a semiconductor laser, has been tried to be used as a light source. The laser source has a longer lifetime and does not require a maintenance work for a long time. Moreover, because the laser source can be directly modulated in accordance with an image to be displayed, the configuration is simplified and the light use efficiency is improved. In addition, a color reproduction range can be widened by employing the laser source.
A laser source has a high coherence. Therefore, when a laser source is adopted as a light source in a projection-type display apparatus, a light diffusing material within a transmission-type screen interferes with light and thus the displayed image glares (speckle noise or scintillation). Such interference leads to degradation of the picture quality. Thus, when the projection-type display apparatus employs a laser source, it is important to reduce the scintillation and speckle noise. A method for reducing scintillation and speckle noise has been proposed, in which a diffusion device is rotated in an optical system (see Japanese Patent Application Laid-open No. H6-208089).
When the projection-type display apparatus that employs a laser source rotates a diffusion device in an optical system to reduce scintillation, frosted glass or the like is used for the diffusion device (diffused plate). However, the conventional art described above does not specifically suggest the configuration and operation of the diffusion device, for example, where the diffused plate is arranged in the optical system or how the diffused plate is rotated. Consequently, there is a problem in that the light use efficiency is degraded.